Glossary
SCR and SNCR
Urea SNCR and aqueous-ammonia SNCR
Also known as urea SNCR, aqueous ammonia SNCR, reagent choice SNCR.
SNCR systems use one of two principal NOx-reducing reagents: urea (CO(NH₂)₂, usually delivered as 32–50% aqueous solution) or aqueous ammonia (NH₃ at 19–29% in water).
Reagent comparison
| Attribute | Urea | Aqueous ammonia |
|---|---|---|
| Storage hazard class | Non-hazardous | Toxic / corrosive |
| Required setback distance | Modest | Large (depends on jurisdiction) |
| Permit complexity | Lower | Higher |
| Reaction rate | Slower (decomposes first to NH₃) | Faster (direct NH₃) |
| Reagent cost per kg-NO removed | Higher | Lower |
| Suitability for cold furnaces | Good | Less good — vaporisation/distribution issues |
| Solid by-product risk | Urea solids at lance tips | None |
Selection drivers
Many plants choose urea for the permitting and safety advantages despite its higher reagent cost; large utilities with established ammonia handling tend towards aqueous ammonia for the lower OPEX. Both reagents produce the same ammonia slip and downstream ammonium-bisulphate consequences when slip is high.
Related terms
Related terms
- Selective Non-Catalytic ReductionSNCR injects ammonia or urea directly into the furnace at 850–1100 °C to reduce NOx without a catalyst. Cheaper than SCR but lower efficiency and higher slip.
- Ammonia slipAmmonia slip is unreacted ammonia leaving the DeNOx system in the flue gas. It is regulated, expensive in lost reagent, and causes ammonium-bisulphate fouling downstream.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.